For a qualitative or quantitative determination of various components contained in a sample, a liquid chromatograph mass spectrometer (LC/MS) is widely used, which is an apparatus including a liquid chromatograph (LC) unit for temporally separating those components and a mass spectrometer (MS) unit for ionizing the separated components and for performing a mass spectrometry of the ions. In many cases, an electrospray ionization (ESI) source or atmospheric pressure chemical ionization (APCI) source is used as the ionization source in the mass spectrometer unit. The ESI source is used in the case where the component to be analyzed is a high-polarity compound, while the APCI source is used for a low-polarity compound.
A sample which contains a plurality of components often contains a mixture of high-polarity and low-polarity components. As a device for performing a mass spectrometry for such a sample, a mass spectrometer equipped with an ionization source called the “dual ionization source” has been proposed, which includes both the ESI source and the APCI source (for example, see Patent Literature 1). A dual ionization source includes: an ESI probe into which a liquid sample is introduced; an ESI power source which supplies the ESI probe with high voltage (ESI voltage); a needle (corona needle), located near the exit port of the ESI probe, for inducing a corona discharge by supplying high voltage; and an APCI power source which supplies the needle with high voltage (APCI voltage).
The liquid sample and mobile phase which have been separated into components in the liquid chromatograph and introduced into the ESI probe are discharged from this probe in the form of droplets which are electrically charged due to the ESI voltage applied to the ESI probe. In this process, high-polarity components are ionized among the components contained in the sample, while low-polarity components contained in the sample are not ionized at this stage; they are ionized by exchanging electric charges with the mobile phase which is ionized by the corona discharge at the corona needle.